The invention relates to an aqueous, acidic coating solution containing chromium for treating zinc, zinc alloy or cadmium surfaces, commonly called a passivating bath.
An acidic chromium containing coating solution for zinc and cadmium surfaces is known e.g. from U.S. Pat. No. 4,263,059 which contains besides trivalent chromium and an acid such as formic acid, acetic acid, propionic acid or nitric acid, sulfuric acid, hydrochloric acid and hydrofluoric acid, a fluoride, the trivalent chromium being formed from hexavalent chromium, such as chromium trioxide, alkali metal chromate and alkali metal dichromate and a reducing agent, such as an aldehyde or alcohol or an alkali metal sulfite, bisulfite, metabisulfite, iodide or of hydrogen peroxide, sulfur dioxide or a bivalent iron salt.
By treatment with this solution the surfaces of zinc, zinc alloy, zinc layers which have been electrocoated or coated by heat onto iron or steel, or surfaces of cadmium are much more corrosion resistant which is indicated by the development of a colour which may reach from blue to black and yellow up to olive green and which is used also for decorative purposes. In particular this coating such as the so-called blue-passivating layer which is a light blue coloured very thin passivating layer of a good decorative appearance inhibits largely the formation of zinc corrosion products also called "white rust".
A measure of the corrosion protection of this chromating is constituted by the salt spray test according to DIN 50021. This test involves subjecting a zinc coated and passivated metal part at 40.degree. C. and 100% relative humidity to a 5% sodium chloride mist. The result is expressed in hours needed to oxidize 5 to 10% of the surface to white rust. The DIN 50021 values for blue passivating corrosion protection should amount from 20 to 24 hours.
The presence of toxic hexavalent chromium compounds in the coating bath of the prior art, however, enables hexavalent chromium to be included into the passivating layer which may lead to dermatitis when handling passivated zinc coated apparatuses e.g. in the food processing industry. Further waste water treatment for removing the remainder of such coating solutions and rinsing waste water, respectively, offers problems. The solutions are rather soon exhausted and can be regenerated only to a limited extent but must rather soon be removed. Corrosion protection obtainable by such solutions decreased within a short time, sometimes even after one day only.
Other acidic coating solutions containing trivalent chromium and also oxidizing agents (U.S. Pat. No. 4,171,231) and further additives, like silicates and/or other metal ions (U.S. Pat. Nos. 4,384,902; 4,359,347; 4,367,099) or organophosphorus compounds (U.S. Pat. No. 4,539,348) or carboxylic acids (U.S. Pat. No. 4,349,392) are suited to form decorative blue and yellow passivating layers but cannot be coloured afterwards and show corrosion test results according to DIN 50021 of a maximum of 6 hours for 10% white rust. Because of the presence of oxidizing agents the layers are not free from hexavalent chromium compounds which are in particular formed when increasing the pH during waste water treatment in the sedimentation step and which make decontamination more difficult.